1. Field of the Invention
The present invention generally relates to a picture stabilizing circuit in a color television receiver. More specifically, the present invention relates to a picture stabilizing circuit acting upon a composite synchronizing signal being applied to a picture associated circuit for obtaining a stabilized color television picture.
2. Description of the Prior Art
A color television receiver includes several circuits associated with the stability and quality of picture of color television and such circuits are generally referred to as "a picture associated circuit" in the present application. A color television receiver is provided with a horizontal automatic frequency control circuit and/or automatic phase control circuit for the purpose of synchronization of a receiver with a transmitter for providing a stabilized picture. A burst gate circuit is also provided for extracting a color burst signal included in a color television signal. A color subcarrier is generated using as a reference signal a color burst extracted by the burst gate circuit. A direct current restoring circuit is also provided for restoring a direct current component of a composite video signal obtained from a video detector circuit. A synchronization detecting circuit is also provided for the purpose of detecting whether or not a color television receiver is in synchronized state responsive to a composite synchronizing signal and a horizontal flyback pulse. If and when not only a horizontal synchronizing signal but also a vertical synchronizing signal and an equalizing pulse are supplied as a reference input to a horizontal automatic frequency control circuit and/or automatic phase control circuit, a picture distortion occurs at the start end of the vertical scanning of the screen. The output of the burst gate circuit provides only a noise component during a period in the absence of the color burst such as during a period of an equalizing pulse and a horizontal synchronizing signal. When such an output of a burst gate circuit is applied to a color processing circuit, degradation of the signal to noise ratio of a reproduced color signal is caused. A direct current restoring circuit often employs a burst gate pulse as a direct current restoring gate pulse. In the case of such a direct current restoring circuit, the restored direct current level is fixed to the level of the peak of such synchronizing signal during a period of a vertical synchronizing signal and a period of an equalizing pulse and therefore a brightness variation is caused in the vicinity of a vertical synchronization period, which accordingly degrades the quality of picture. Furthermore, a synchronization detecting circuit generally utilizes a logic circuit for receiving a composite synchronizing signal and a horizontal flyback pulse. Such a logic circuit comprises an exclusive OR circuit receiving a composite synchronizing signal and a horizontal flyback pulse or an AND gate receiving a composite synchronizing signal and a horizontal flyback pulse. In the case of such synchronization detecting circuit, application of a vertical synchronizing signal and an equalizing pulse as inputs leads to a decrease in sensitivity for synchronizing detection. Thus, application of an equalizing pulse and a vertical synchronizing signal to a picture associated circuit in a conventional color television receiver leads to a decrease in stability of a picture or the quality of a picture as a whole.
In order to eliminate the above described problems, the following approach may be considered. More specifically, one approach is to render ineffective or to mask the composite synchronizing signal being applied to the picture associated circuit during an equalizing pulse period and a vertical synchronizing signal period. In order to make such a rendering ineffective rendering operation, it is necessary to generate an ineffective rendering pulse. In order to generate such an ineffective rendering pulse, one might simply think of employment of a one-shot multivibrator so that a pulse may be generated using a vertical synchronizing signal from one field before as a reference so as to cover a vertical synchronizing signal period and an equalizing pulse period of the subsequent field. However, it is much difficult to delay an ineffective rendering pulse generated by such one-shot multivibrator by one field with accuracy irrespective of a temperature change. Accordingly, the above described obvious approach is lacking in temperature stability and is not practicable.